Tube for tube heater



1968 K. w. FLEISCHER ET AL 3,399,117

TUBE FOR TUBE HEATER Filed Nov. 16, 1966 FIG. 2

E M G M m R m NEE VCC NSW E L w T Q :2 U0 KR Y B ATTORNEY.

United States Patent ABSTRACT OF THE DISCLOSURE A reforming furnacehaving tubes made in sections of successively higher alloys as the fluidbeing treated is subjected to increasingly severe heating.

The present invention relates to liquid heating furnaces, and moreparticularly to the tubes through which the liquid to be heated flowsand the manner in which the tubes are fabricated.

In modern tu-be heating furnaces, and particularly those used in thepetrochemical industry for the making of ethylene and the like, themetal used for the tubes is of material importance. The tubes must havechemical and physical characteristics that are capable of withstandingthe maximum temperatures and pressures that are encountered. Usually themaximum stress imposed on a tube is at its exit end where the tube andliquid being heated are hottest. The entire tube, however, has to becapable of withstanding this maximum severity condition even though itmay occur in only the last third or fourth of the tube.

The cost of the tubes used in a furnace with severe heating requirementsis a large portion ofthe cost of a furnace. This is both because of thelength of the tubes, that may be up to sixty feet long, and because theymust be of high alloy steel to withstand the service. Anything that canbe done to reduce the cost of the tubes is of importance in theconstruction of such equipment.

It is an object of the invention to provide a means of lowering the costof the tubes used in a fluid heating furnace. It is another object ofthe invention to provide a tube that is made of different materialsalong its length, which materials vary with the temperature to whichthat portion of the tube is to be heated or the severity of the service.

The various features of novelty which characterize our invention arepointed out with particularlity in the claims annexed to and forming apart of this specification For a better understanding of the invention,however, its advantages and specific objects attained with its use,reference should be had to the accompanying drawings and descriptivematter in which we have illustrated and described a preferred embodimentof the invention.

In the drawings:

FIG. 1 is a section through one type of furnace in which a tube of theinvention can be used; and

FIG. 2 is a view through a different type of furnace.

The furnace disclosed in FIG. 1 is of the type that is disclosed more indetail in Fleischer Patent 3,062,197. A furnace of this type includes astructure 1 forming a furnace chamber having parallel side walls 2 and 3and having an exhaust opening 4 at the upper end in one of the sides.The furnace is heated by horizontally extending parallel rows of burners5 with the rows located one above the other in the side walls. Theburners are directed toward a row of tubes, one of which is shown at 6,that extend vertically through the furnace chamber. Each horizontal rowof burners 5 will be fired at a rate to raise the temperature of theportion of the tube in front of it to the desired value. The fluid to beheated which for example, if the product is to be hydrogen, can be acombination of methane and steam, will be directed vertically throughthe tube from either the top or the bottom. The tube, for reasons to bedescribed below, is made of a plurality of sections 7, 8 and 9 weldedtogether at joints 10.

The furnace shown in FIG. 2 is constructed similar to that of FIG. ,1except that it is a dual furnace having two chambers one of which isindicated at 11 and the other at 12. These chambers are provided withexhaust passages 13 and 14 respectively that lead to a convectionheating chamber 15 at the base of a stack. As is frequently the case,the exhaust gases flowing to the stack flow past a group of tubes 16 inchamber 15, known as a convection section, in which the tubes are heatedby convection from the exhaust gases. The material flowing through thetubes of convection section 16 is directed to tubes 17 which are locatedin the chambers formed in furnace sections 11 and 12. The arrangementcan be such that the material flowing through convection section 16 cango to a manifold which in turn supplies the various tubes 17 or it cango through a plurality of connecting pipes 18 directly into each of thetubes 17. The manner in which the piping is fabricated will dependentirely upon the operation which is to be performed by the furnace. Inany event the fluid to be treated is preheated in the convection sectionand is directed to tubes 17 in the two sections 11 and 12 of the furnacewhich are generally known as the radiant sections. In this section thefluid is raised from its preheat temperature to the final treatmenttemperature. As shown herein, the tube 18 is welded to the upper end oftube 17 and this latter tube is made up of two sections 19 and 21 thatare joined together by a weld 20.

The materials of which the tubes are fabricated will depend upon theseverity of the service and the temperature to which the tubes and thematerial flowing through them is heated. The cost of the tubes will varywith their composition. It is almost axiomatic, however, the more severethe service and the higher the temperature and pressure required, themore expensive the tubes.

If, for example, the material to be heated is introduced into the tubeat about 700 F. and discharged at 1600 F. the tube will have atemperature gradient of from about 1000 F. to 1850 F. A tube compositionwhich is satisfactory for 1000 F. service is completely unsatisfactoryfor 1800 F. service. Prior to this time the entire tube has been made ofa material capable of operating satisfactorily at the highesttemperature encountered.

According to this invention the tubes are made in sections, and weldedtogether, with each section of an analysis capable of withstanding thetemperatures encountered along that portion of its length. Examples ofmaterials that will withstand various temperatures are:

Carbon steel for temperatures below 900 F.

Alloy steel with 1.25% Cr and 0.5% M0 for use from about 900 F. to about1150 F.

Alloy steel with 18% Cr and 8% Ni for use to about 1600 F.

Alloy steel with 25% Cr and 20% Ni for use from about 1600 F. to 1800 F.

Alloy steel with 15 %-20% Cr and 35% Ni for use to about 1800 F.

Alloy steel with Ni and 15% Cr for temperatures to above 2000 F.

It will be noticed that the higher the temperature, the higher the alloyused. There are other alloys that are available for use within thetemperature ranges mentioned and new alloys are being developed fromtime to time that can 'be made into tubes for various serviceconditions. If their properties are suitable they may also be used.

The temperature of tube and the temperature of the m'at'erial'in itcan'be determined at any point along its length by conventional methods.When this has been done, the proper alloy for any particular locationcan be determined. Ordinarily it will not be advisable to have the tubemade of more than three sections for a tube forty feet in length. a

The furnace or heater of FIG. 1 shows a tube fabricated of threesections each of which will be from ten to fifteen feet in length for aforty foot tube. The direction'of flow of the fluid being heated willdetermine whether the more expensive alloy is the top or bottom section.The tube 17 in FIG. 2 is shown as being of two sections. In a heater ofthis type the fluid is heated in convection section 16 to about 900 P.so that the convection coil and connecting portion 18 will be made ofcarbon steel tubes. For the higher temperatures in chamber 11 alloyssuitable for the temperatures encountered will be used.

Thus it will be seen that the amount of expensive alloy used in a tubecan be materially reduced by matching the alloy to the temperatureencountered.

While in accordance with the provisions of the Statutes we haveillustrated and described the best form of embodiment of our inventionnow known to us, it will be apparent to those skilled in the art thatchanges may be made in the form of the apparatus disclosed withoutdeparting from the spirit and scope of the invention set forth in theappended claims, and that in some cases certain features of ourinvention may be used to advantage without a corresponding use of otherfeatures.

'What is claimed is:

1. In a tube heating furnace for the pyrolysis of hydrocarbons, thecombination of structure forming a furnace "chamber having opposedwalls, a plurality of vertically spaced burners located in said walls, atube extending vertically through said chamber between said walls and infront of said burners to be heated thereby, said tube being adapted totransport fluid to be raised in temperature as it flows from first endto asecond end, said tube being made in a plurality of end to endsections with each section in the direction of fiuid flow beingmade of adifferent metal capable of withstandingsuccessively higher temperaturesand-morefsevere service than the metal in the preceding section.

2. The combination of claim 1 in which said tube sec tions are made ofdifferent alloys and are welded together.

3. The combination of'claim' 1 in which "one of said walls isprovide'dwith an exhaust opening through which products of combustionescape, means forminga convection chamber in communication with'said'exhaust opening, a convection coil in said convection chamber, andmeans connecting said coil with said first end of said tube.

4. The combination of claim 3 in which said convection coil is made of ametal capable of withstandinga lower temperature than. the metals usedin said tube sections. I

References Cited Guerrie ri 122-35 6 CHARLES J. MYHRE, Primary Examiner.

